Electric braking with alternating-current motors.



W/'nesses-' f., Y fnventor:

fav( Ernst/l/l/.ff/exandersor. @y y /l/Qf/'Attorney ELECTRIC ,BBAKING WITH ALTERNATING G APPLICATION FILED 00T. 19, 17911.

' Patented Mar. 3, 1914 i Electric Braking with Alternating-Current -produce the ,undesirable effects mentioned its magnetization.

UNITED STATES PATENT oEEioE.

ERNST F. W. ALEXANDERSON, 0F SCHENECTADY, NEW YORK, ASSIGNOR TO -GENERAL ELECTRIC COMPANY, A CORPORATION 0F NEW YORK.

ELECTRIC ERAvKING WITH'. ALTERNATINGr-CURREN1 MOTORS.

Specication'of Letters Patent.

Patented Mar. 3, 1914.

Application filed October 19, 1911. Serial N o. 655,477.

T 0 all whom 'it may concern Be it known that I, ERNST F. W. ALEX- ANQERSON, a citizenof the United States, residing'at Schenectady, in the county of Schenectady, State of New York, have made certain new and useful Improvements in Motorspof which the following is a specification.

My invention relates to alternating current commutator motors fof the series type, that is, motors having an exciting winding adapted for connection in series with the armature for motor operation, and has for its purpose the provision of a suitable arrangement for operating such motors as braking generators.

A difliculty which has been encountered heretofore in operating such motors as braking generators is that the motor when connected for braking tends to build upon its residual magnetism as a direct current series generator. 'Ihe ohmic resistance of themotor circuit is-ordinarily low so that excessive braking action is produced. It has been proposed heretofore` to avoid this effect by connecting the motor armature and exciting winding in series through a transformer, instead of directly, so as to prevent thebuilding up of a direct current and Such an arrangement, however, is not satisfactory with a transformer" ofthe ordinary type because, al-v though operation as a direct current generator is prevented, nevertheless the motor may build up as a low frequency generator giving strong pulsations which are unstable and excessive.

My invention consists in an arrangement whereby` the low frequency pulsations are avoided.A To this end I employ a transformer of aspecial and inefficient design for connecting the armature and exciting windings in series. This transformer is so ar-v ranged as to require an abnormally high magnetizing current, as by placing an air gap in the magnetic circuit of the transformer. The result is that a small low frequency current in the primary of the transformer is not eective for creating an induced secondary voltage large enough to above, while the higher frequency currents, that is, currents of the frequency of the source, will produce a correspondingly higher voltage in the transformer secondary which supplies the exciting winding of the motor. Y

My invention will best be understood by reference to the accompanying drawings, in which- Figure l shows diagrammatically an arrangement for electric braking in accordance with my invention, and Fig. 2 shows a modification.

In Fig. l A represents the armature, B .the exciting winding and C the compensating Winding of an alternating current series motor; D represents a supply transformer acting as the source of current for motor operation, and receiving the current generated by the motor in electric braking; E represents a transformer having its primary winding connected in series with the motor armature A and compensating winding C to the transformer D, and its secondary winding connected to the exciting winding B.

0f course; when the motor is used for electric braking, the exciting winding B is connected to the secondary winding of the. transformer E in such a direction as to produce a generator torque, or, in other words, a torque opposite to the direction Of rotation. This transformer E is arranged with an air gap in its magnetic circuit so as to require an abnormally high magnetizing current for effective operation. The'etliciency of the ltransformcr may in many cases also be advantageously further decreased by providing a leakage path for the primary flux as is indicated by the unwound third leg of the magnetic circuit of the transformer in Fig. l. The effect of this ineiiicient design, as has been heretofore explained, is to render the low frequency currents which may be generated in the motor armature AcomparativehY ineffective to excite the winding B. while the relative effect of currents of the frequency of the source is very much greater. The low frequency cur` i .rents can further be controlled by employing non-inductive resistances F in either or both circuits of the transformer E. i

Instead of using resistanccs to augment the effect of the ineihcicnt design of transformer E, the arrangement of Fig. 2 may be employed. In this gnre a portion of the exciting winding B is included in circuit with thc armature A and the primary lof transformer E, while the secondary of transt'oianer E is connected in series with the Whole of Winding l.. The connections are so made that the current owing through the portion'o Winding B in series with armature A is in opposition to the current supplied to the Whole Winding from the second ary of transformer E. At low frequency the portion of the exciting Winding in series with the armature counterbalances in Whole or in part the comparatively Weak currents delivered to the Whole Winding by the secondary of transformer E, While at higher frequency the transformer E is relatively more eicient so that the current which its secondary supplies to winding B is relatively considerably greater in its magnetizing effect than the current in the portion of the Winding in series with the armature.

' I do not desire to limit myself to the par-v ticular connections and arrangement of parts shown and described but aim in the appended claims to cover all modifications which are Within the scope of my invention. 'What I claim as new and desire to secure by Letters Patent of the United States is l. In combination an alternating current series motor provided with means whereby a motor action or a braking generator action can be produced, the means for operating the motor as a braking generator comprising a transformer requiring an abnorinally high magnetizing current so that the motor Will not build up as a low frequency generator, and leads connecting the motor armature and transformer primary in series to the source and the transformer secondary to the exciting Winding of the motorin a direction to produce a torque oppo site to the direction of rotation.

combination an alternating current a incassa series motor provided With means whereby a motor action or a braking generator action can be produced, the means for operating the motor as a braking generator comprising a transformer having an air gap in its magnetic circuit, and leads connecting the motor armature and transformer primary in series to the source and the trans former secondary to the exciting Winding of the motor in a direction to produce a torque opposite to the direction of rotation.'

3. In combination with an alternating current series motor, means for operating the motor as a braking generator comprising a transformer' requiring an abnormally high magnetizing current so that said motor Will not build up as a loW frequency generator, and leads connecting the motor armature, a part of the exciting Winding of the motor, and the transformer primary to the source and the transformer secondary in series with the whole of said exciting Wind. ing.

4, In combination with an alternating current series motor, means for operating the motor as a braking generator comprising a transformer having an air gap in its,

magnetic circuit and leads connecting the motor armature, a part of the exciting Winding of the motor,` and the transformer primary in series to the source, and the transformer secondary in series with the EWhole of said exciting Winding.

In Witness whereof, I have hereunto set my hand this 17th day of October 1911.

ERNST F.- W. ALEXANDERSON.

Witnesses:

BENJAMIN B. HULL, HELEN ORroRD. 

